The invention relates to a device for measuring the level of liquid in a container, particularly a fuel tank or the like, which contains a temperature-dependent electric resistor which is adapted to be heated by a suitable means and utilizes a measuring device which indicates changes in the resistance value as a result of change in the fill level of the liquid.
A measuring device for reading liquid levels is known from German AS No. 23 08 823, in which is disclosed a plurality of sheath conductors, of which one part includes resistance wires and heat conductors carrying current, and another part contains only resistance wires wound on a carrier bar. Such resistance wires are arranged in a measuring bridge circuit, whereby the temperature-dependent resistance wires of a heated sheath conductor are arranged in one bridge arm and those of an unheated sheath conductor are arranged in another bridge arm of the resistance measuring bridge. This type of measuring device for indicating the level reading, is particularly adapted for use in measuring liquid levels in nuclear reactor power plants where, as a rule, the level of water is to be measured.
Such devices, however are not suitable for the measurement of liquid levels of fuels or other readily volatile compounds. The mass of the measuring body to be heated is very large in relationship to its surface which can be contacted by the liquid. This produces not only a great inertia in the display operation and a low precision of measurement, as a result of the heat dissipation in the measuring body, but in addition a relatively high heat output must be expended because a significant temperature gradient exists within the measuring device, and only the mean temperature difference between the part of the measuring device disposed in the liquid and the part disposed outside of the liquid can be evaluated to provide a measurement. Consequently, the part disposed outside of the liquid is relatively strongly heated and can produce a vaporization of the liquid at the boundary area. This results not only in the loss of liquid due to vaporization, but also where employed in connection with combustible liquids, results in an increased danger of explosion. Moreover, as a result of the necessary heat output and the low heat conductivity of fuels, in such a measuring device, a heating occurs in the part of the resistor lying within the liquid that greatly weakens the signal.
It has also been proposed to employ the change in capacitance between two neighboring electric conductors or conductor surfaces in connection with the determination of the height of a liquid level. Such methods involve a relatively high outlay for apparatus and are susceptible to contamination during operation. Further, the influence of unavoidable supply line capacitances on the measuring result is extremely difficult to suppress.